The present invention relates to a sheet feeding apparatus and, more particularly, to a sheet feeding apparatus for feeding out mail items having different sizes, weights, and shapes one by one.
As an example of a conventional sheet feeding apparatus, a sheet extracting apparatus described in Japanese Patent Laid-Open No. 6-71227 (Reference 1) is known.
Reference 1 describes a technique for extracting sheets by utilizing the negative pressure of a negative pressure chamber. Reference 1 also describes a technique for measuring the pressure in the negative pressure chamber with a pressure sensor and informing an abnormality when the value of the pressure sensor becomes less than a predetermined value, and a technique for blocking the flow of air to the negative pressure chamber with a relief valve connected to the negative pressure chamber, when no mail exists, to decrease noise.
FIG. 4 shows the schematic arrangement of the conventional sheet extracting apparatus described in Reference 1. The sheet extracting apparatus shown in FIG. 4 comprises a suction belt 60 for supplying a mail item 58, a negative pressure chamber 62 close to the suction belt 60 to receive a negative pressure, an air pipe 64 connected to the negative pressure chamber 62, a relief valve 66 for controlling the negative pressure of the negative pressure chamber 62, an air filter 68 for removing dust and the like in the air, and a blower 70 for generating a negative pressure.
A pressure sensor 72 for detecting the pressure is placed in the negative pressure chamber 62, and a mail sensor 74 for detecting the presence/absence of a mail item is placed at a position where the mail item 58 is placed. Outputs from the sensors 72 and 74 are input to a CPU (Central Processing Unit) 76 having an air pressure determining section 76a. The CPU 76 is connected to a valve controller 78 for controlling the relief valve 66, and a display 82 and a voice guide 84 that constitute an informing unit 80 for informing the operator of the state of the apparatus.
The operation of the sheet extracting apparatus having the above arrangement will be described.
Usually, the mail items 58 are drawn by the suction belt 60 with the negative pressure of the negative pressure chamber 62 one by one, and each mail item is fed by the suction belt 60 in the convey direction. With a lapse of a predetermined period of time since the start of operation of the apparatus, if the mail sensor 74 determines that no mail item 58 has been fed to the suction belt 60 for a predetermined period of time or more, the CPU 76 controls the valve controller 78 to diagnose the apparatus by utilizing the idling time during which no mail item is fed. Thus, the relief valve 66 is closed to set the interior of the negative pressure chamber 62 at a negative pressure.
The state in the negative pressure chamber 62 is detected by the pressure sensor 72. The CPU 76 compares the detected pressure and a normal-state pressure with the air pressure determining section 76a. The CPU 76 determines whether the comparison result is less than a predetermined value. If the comparison result is less than a predetermined value, the CPU 76 drives the display 82 and voice guide 84 of the informing unit 80 to inform the operator of an abnormality.
From the detection result of the mail sensor 74, if no mail item 58 is placed in front of the suction belt 60, the relief valve 66 is switched to an open state to release the pressure, so the interior of the negative pressure chamber 62 does not become a negative pressure. This aims at preventing the interior of the negative pressure chamber 62 from becoming a negative pressure to generate air suction noise.
In the conventional sheet extracting apparatus described above, when sheets such as mail items having largely different sizes, weights, and shapes are to be extracted by suction one by one, sometimes a particularly heavy mail item is not reliably drawn by suction with the rotating suction belt 60. In this case, the suction belt 60 performs idling and does not feed a mail item, or a mail item is conveyed while slipping on the suction belt 60. Then, an interval cannot be maintained between the currently fed mail item and a mail item which is to be extracted next.
This is because, when feeding the mail, the suction belt 60 is continuously rotated at a constant speed without stopping. Another reason is as follows. The pressure in the negative pressure chamber 62 is measured by the pressure sensor 72. If the measurement result is less than a predetermined value, it is utilized for only informing the operator of the abnormality of the suction portion including the negative pressure chamber.
In the conventional sheet extracting apparatus described above, when one mail item is extracted and thereafter the next mail item is to be extracted, the next mail item is sometimes fed at an earlier timing than usual. In this case, the next mail item overlaps the current one to be conveyed together with it.
This is due to the following reason. Since the relief valve 66 is not turned on/off every time one mail item is fed, the interior of the negative pressure chamber 62 is always at a negative pressure, and accordingly a force for drawing the mail item by suction is always effected.